It is possible that my choosing of words is wrong or I’m mixing up British and American English, because English is a foreign language to me. I know a Canadian that says that too and he is not from Quebec, but I’m getting of topic here.
After reading the High Guard book I noticed that the fuel consumption and size of Jump Drives has no technological progression, which it had in GDW’s “Megatraveller” and “The New Era” Traveller rules.
Since I couldn’t find any information in this forum regarding the “old” Jump Drive rules I decided to provide them for Mongoose Traveller 2nd Edition.

Quick Overview:
Jump Drive+ (TNE): An adaptation of the Traveller “The New Era” Jump Drive construction to HIGHGUARD rules.
Hybrid Jump Drive: A Jump Drive that has a very small high tech core for increased Jump Range, supported by low tech components. This is the HIGHGUARD Jump Drive.
Linear Jump Drive: A Jump Drive that has a linear time progression while in Jump Space instead of the fixed one week.

JUMP Drive+
Its a Jump Drive nothing special to describe. I know the name is stupid, but I needed to differentiate otherwise two legged mammals get confused

Construction Rules:
All limitations from HIGHGUARD regarding Jump distance still apply.
First take a look at the HIGHGUARD Jump Potential table (p. 14). Every Rating in that table has a related TechLevel.

1. Choose the TL for your drive

2. Calculate the “% of Hull” of the drive for a 1 parsec jump. The formula to do this is:
% of Hull for 1 parsec = (1 + Jump Rating) / Jump Rating'
For example: % of Hull for 1 parsec of a TL14 Jump drive, would be 1,2 ((1+5)/5)

3. If you want to Jump further than 1 parsec, multiply the value from step 2 by the desired Jump Range to get the final Jump Drive size.

5. Calculate Fuel requirement
Multiply the Jump Drive size from step 4. by the multiplier from this table (should be one but couldn't figure how to do it, just ignore the __), to get the fuel size in ship tons.
TechLevel 9-16_17__18__19__20__21
Multiplier x5___x4__x3__x2__x1__x0.5

The minumum size for a Jump Drive+ is 0,5 tons.

Cost: Jump Drive+ cost MCr 2.5 per ton

Impact on the game: Small ships can have more space on their vessel, resulting in better trade revenue. Depending on the style your gamemaster likes to run the game, that might cause some issues. Personally I like the idea that the players ship can be individualized with labs, a grav bike garage etc. and still make good money.

Hybrid Jump Drive
For the Hybrid Jump drive to work the following assumptions have been made.
Only a very small part of a Jump Drive the Jump Core is responsible for the Range, while the rest of the drive is just fuel converters, energy banks, jump buffers/dampeners, cooling systems, hull jump grid etc. This is not really a new drive system it is more intended as a bridge to keep all the HIGHGUARD ship designs valid, while still allowing the inclusion of the Jump Drive+ in your game. It isn’t a perfect solution, for example there couldn’t be a TL 9 Hybrid Drive for a TL 9 society. Think of it in that case of a more robust easier to maintain bulkier version of the JumpDrive+ or a less refined one.

Well if the Hybrid Jump Drive is just there to keep the HIGHGUARD rules valid, why are there construction rules for it?
Glad that you asked (doing a MightyJingles on you, aren’t I). Even with the increased cost of the Jump Drive+ at some point it still gets less expensive than the HIGHGUARD Jump Drive (which is a TL9 Hybrid Jump drive), so we need a way to upgrade it to a higher TL base.

Construction Rules:

All limitations from HIGHGUARD regarding Jump distance still apply.

1. Choose the TL for your drive; must be at least one level lower than the highest available (e.g. TL 14 for TL 15 Shipyard; does not apply for TL9)

2. Calculate the “% of Hull” of the drive for a 1 parsec jump. The formula to do this is:
% of Hull for 1 parsec = (1 + Jump Rating) / Jump Rating
For example: % of Hull for 1 parsec of a TL14 Jump drive, would be 1,2 ((1+5)/5)

3. Calculate Fuel requirement for a 1 parsec jump
Multiply the % of Hull for 1 parsec from step 2. by the multiplier from this table, to get the Jump Fuel
TechLevel 9-16_17__18__19__20__21
Multiplier x5___x4__x3__x2__x1__x0.5
for a 1 parsec Jump.

4. Add 0.5 to the result from step 2 to get final Drive size per parsec.

5. If you want to Jump further than 1 parsec, multiply the value from step 4 by the desired Jump Range to get the final Jump Drive size.

What the <I am to British for that> !? This is even more complicated then the Jump Drive+
I hear you, so for the lazy folk here is a table:

TechLevel________ 9___11___12____13____14___15___16
% of hull per parsec 2,5__2____11/6__1.75__1.7__5/3__23/14
fuel per parsec
in % of hull________10__7.5___20/3__6.25__6___35/6__40/7

Impact on the game: None.

Linear Jump Drive
This one is for people that want iconic vessels like the Millennium Falcon in their Traveller setting without braking the basics of the setting like message distribution speed based on Jump Rating. It breaks it a little on the local subsector level but doesn’t change it for Empire scale entities. A Linear Jump Drives rating is also its speed in parsec per week. It stays in Jump Space based on that speed. For example a Jump 6 vessel doing a 1 parsec jump would emerge at its destination after 7/6 (1 week / Jump Rating) days. The maximum time in Jumpspace of 1 week still applies. This one is top notch military grade hardware, you need friends in high places (permit) to get your hands on that one, or build it yourself.

Construction Rules:
Identical to Jump Drive+, but you are not required to carry enough fuel for your maximum jump distance.
Cost: Linear Jump Drive cost MCr 3,5 (this is a 40% increase over the Jump Drive+) per tonNote: Cost might have to be increased, to reduce the amount of warships using that drive.

Impact on the game: Negligible on sector and empire scale level, but noticeable on subsector level. News can travel faster than a week from an XBoat stop to neighboring systems, advanced Patrol Cruisers/Bounty Hunter vessels can catch up/overtake a pursued vessel. Large scale Naval warfare would use specialised fast reaction reserve fleets with this drive; Couriers to get more up to date information from neighboring systems.

Before anyone else thinks about this I should better address this.Can a Linear Jump Drive do a jump inside a star system?
NO and YES, depends on how complicated you want to make your RPG life. If you stick to answer NO, hooray we are finished here.
And now I’ll have to feed the insatiable hungry fish (why do I think of Piranhas all of a sudden) YES we want faction.
First we need to establish how a jump works. Unfortunately the only book where this was ever explained I believe was the out of print hard to come by Starhip Operator Manual for Megatraveller, which a friend of mine has and I read at least once. If I remember correctly entering Jump Space requires 60% of your jump fuel, 20% is expended during travel and then another 20% for reentry into normal space. If we ignore the negligible fuel expense during travel, that still leaves us with 80% of fuel that a 1 parsec jump would need for an in-system jump. Also Entry and Reentry require a different setup in the Jump Drive, meaning that at least a minute should be between Entry and Reentry to allow for reconfiguration of the drive. So the minimum distance traveled would be 60 multiplied with 170 times the speed of light per second. By the way the NO faction stopped reading five minutes ago, just saying. If the Jump Drive is sturdy enough and there is still fuel you could even do multiple in-system jumps in short succession. This is quite simple to calculate, the maximum input energy a drive can take is the entry into J-Space, for a jump 2 drive that would be 120% of a jump 1 drive. A second jump would mean that your drive must be able to survive the equivalent of 160% (2 times 80%) of a jump 1 drive. So a jump 2 could not while a jump 3 (180%) drive could. In-system jumps, if you allow it in your Traveller universe, are considered last ditch emergency procedures on military vessels that are equipped with a Linear Jump Drive to save the ship. In other words, Jump Drives don’t like to be treated that way (multiple jumps in short succession) and the risks (at your GM’s discretion) of doing so should be accordingly.

ClosingI think all three drive systems can exist in your Traveller setting side by side to enrich your play experience. If you are unsure about the Linear Jump Drive, than build an adventure around it, where your group is followed by a vessel equipped with it and test how it plays out. If your group likes it, then go with it, otherwise it was just a way to expensive prototype.

Undoubtedly, there are aspects of the jump drive that can be constructed at lower technological levels, but the assumption is that the guts of the device, can only be manufactured at the minimum stated technological level, though the ancillary machinery, such as tanks, fuel purifiers, and pumps, don't need to be.

Exception being prototypes.

With jump drives, lowest common denominator plays a significant factor: if you replace parts with those manufactured from a lower technological level, performance, that is jump factor, is limited to that as permitted by that technological level.

I have allowed several variants like you describe in my games at different times.

One I particularly liked that you don't mention, works with the old LBB system, but not HG.

Double Drives:
You can mount more than one drive (different drive letters) and combine their effect (maximum is still 6). This gives you an installed spare basically and was used in the Annic Nova adventure. Each drive must be able to operate the entire ship at Rating 1, but their combined Rating can exceed 6, but you can only use it at Rating 6. So, you could have a J-4 and J-3 drive, but could only use it at J-6 Maximum. BUT, you get 7 hits before your drive it completely gone.

Works really nicely on military ships where drive numbers are reduced with hits. Having 2 drives gives you a way to spread out the damage and keep fighting longer.

One I particularly liked that you don't mention, works with the old LBB system, but not HG.

Double Drives:
You can mount more than one drive (different drive letters) and combine their effect (maximum is still 6). This gives you an installed spare basically and was used in the Annic Nova adventure. Each drive must be able to operate the entire ship at Rating 1, but their combined Rating can exceed 6, but you can only use it at Rating 6. So, you could have a J-4 and J-3 drive, but could only use it at J-6 Maximum. BUT, you get 7 hits before your drive it completely gone.

The Annic Nova made it into the 2nd edition High Guard and has two drives, you just can't combine them. However you could combine them by using the breakaway hull option.

Undoubtedly, there are aspects of the jump drive that can be constructed at lower technological levels, but the assumption is that the guts of the device, can only be manufactured at the minimum stated technological level, though the ancillary machinery, such as tanks, fuel purifiers, and pumps, don't need to be.

Exception being prototypes.

With jump drives, lowest common denominator plays a significant factor: if you replace parts with those manufactured from a lower technological level, performance, that is jump factor, is limited to that as permitted by that technological level.

However, you are allowed to create your own jump drive variants.

I think there is a little misunderstanding. The nomenclature of a hybrid drive should be for example "Jump-6 TL15/9". 15/9 meaning it is a TL 15 drive(core) with a TL9 support infrastructure. You take the support infrastructure value to select the entry from my table (Hybrid Drive construction rules after step 5) for drive size and fuel consumption. Multiply those by the jump rating to get the final drive size and fuel requirements. This would be the Jump-6 drive from High Guard.
The only reason I invented the Hybrid Drive was to eliminate the need for players and gamemasters to make any changes to deck plans and stats of existing and future vessel designs from Mongoose unless they want to. As long as you don't use Jump Drive+ rules in your game you don't need the Hybrid Drive anyway.

Regards,
Zurbaron

p.s. my first idea instead of the Hybrid Drive was a lengthy text with tips on how and when you need to change deckplans and stats. I think the Hybrid Drive is the cleaner solution, than lots of text and painting.